The space shuttle Atlantis, bolted to a mobile launch platform atop an Apollo-era crawler-transporter, was hauled to launch pad 39A at the Kennedy Space Center on Tuesday for work to ready the ship for blastoff May 12 on a fifth and final mission to service the Hubble Space Telescope.

Originally scheduled for launch on October 14, the long-awaited Hubble overhaul was delayed when one channel of a critical data-processing system unit aboard the telescope failed just two weeks before liftoff. NASA managers decided to replace Hubble's entire science instrument command and data handling unit, or SI/C&DH, to restore redundancy and improve reliability.

But testing a spare ground unit at the Goddard Space Flight Center in Greenbelt, Md., getting it certified for flight, and working the mission back into NASA's shuttle manifest ended up delaying Atlantis and Hubble Servicing Mission 4, or SM-4, for seven months, when all was said and done.

The replacement SI/C&DH was delivered to the Kennedy Space Center on Monday, and Atlantis, attached to an external fuel tank and two solid-fuel boosters, took its first step toward space with a six-and-a-half-hour, 3.2-mile trip from the Vehicle Assembly Building to pad 39A on Tuesday.

Shuttle Atlantis moved to pad 39A for May 12 launch.
William Harwood

Shuttle commander Scott Altman, pilot Gregory C. Johnson, flight engineer Megan McArthur, and spacewalkers John Grunsfeld, Michael Massimino, Andrew Feustel, and Michael Good plan to fly to Kennedy late this week to inspect the replacement computer unit before it is moved to the pad April 18, along with the rest of the Hubble payload, for installation in Atlantis' cargo bay.

Hubble SM-4 is the fifth and final planned shuttle mission to the space telescope (SM-3 was spread across two flights). During five back-to-back spacewalks, the Atlantis astronauts plan to install a new camera, called the Wide Field Camera 3, the Cosmic Origins Spectrograph, a full set of batteries, six new stabilizing gyroscopes, a new fine guidance sensor, new insulation, and to carry out repairs on two other science instruments that are currently out of action.

The new 135-pound science instrument command and data-handling unit will be wired into Hubble's electrical system during the first spacewalk, after the Wide Field Camera 3 is installed.

As it now stands, no major mission objectives have been deleted, despite the late addition of the SI/C&DH installation. But to get everything done, the astronauts must be able to complete a complex repair of the Advanced Camera for Surveys during a single spacewalk. The original flight plan broke that task into two parts.

If it turns out the astronauts need more time for the repair work, the fine guidance sensor replacement could be deleted.

There are no technical problems of any significance with Atlantis or its payload, but analysts are still evaluating the threat posed by orbital debris at Hubble's 350-mile-high altitude. Because of a satellite collision in February, the debris environment is somewhat worse at Hubble's altitude and, as of this writing, the mean chance of a catastrophic impact during the shuttle visit is believed to be around 1 in 185.

Odds worse than 1 in 200 require an executive-level decision on whether the additional risk is acceptable. Engineers say additional analysis, possible changes to the shuttle's orientation in space, and other factors are expected to improve those odds, and senior managers appear confident that Atlantis ultimately will be cleared for flight.

Shuttle program managers plan to meet April 20 and 21 to review launch processing, followed by an executive-level flight readiness review April 30 at the Kennedy Space Center to formally clear the ship for launch. If no problems develop, Atlantis' countdown will begin May 9 for a launch attempt the afternoon of May 12.

The last published launch time was 1:21 p.m. EDT, about 20 minutes into the Hubble launch window. But flight planners may adjust that, pending additional analysis of payload weight and ascent performance margin.

Here is a brief overview of the crew's flight plan (assumes a launch at 1:21 p.m. on May 12; spacewalks, or EVAs, would begin around 6:46 a.m. each day):

Because the Hubble Space Telescope is in a different orbit than the that of the International Space Station, the Atlantis astronauts cannot seek safe haven aboard the lab complex, if a major problem develops that might prevent a safe re-entry.

As a result, NASA plans to move the shuttle Endeavour to launch pad 39B on April 17 to ready the ship for a quick-response blastoff on an emergency rescue mission, if needed. If not, Endeavour will be moved to pad 39A, after Atlantis lands for normal processing, and launch around June 13 on the next space station assembly mission.

About the author

Bill Harwood has been covering the U.S. space program full-time since 1984, first as Cape Canaveral bureau chief for United Press International and now as a consultant for CBS News. He has covered more than 125 shuttle missions, every interplanetary flight since Voyager 2's flyby of Neptune, and scores of commercial and military launches. Based at the Kennedy Space Center in Florida, Harwood is a devoted amateur astronomer and co-author of "Comm Check: The Final Flight of Shuttle Columbia." You can follow his frequent status updates at the CBS News Space page.
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